Case Study 2: Digital Micromirror Devices (DMD) Optical MEMS

Transcription

1 Case Study : Digital Micromirror Devices (DMD) Chapter of Senturia A MEMS-based projection display, Van Kessel, P.F.; Hornbeck, L.J.; Meier, R.E.; Douglass, M.R., Proc. IEEE, Vol. 86 pp Other Literatures on Course Website M. C. Wu 1 Optical MEMS MEMS are well-suited for interaction with light Structural dimensions ~ wavelength Small displacement has large effect (e.g., ON-OFF switching) Interferometric devices : d ~.5 λ Scanning devices : θ ~ a few degrees Photon has no mass Does not need large-force actuators MEMS enables large-scale systems E.g., 1x1 display or optical switches M. C. Wu

2 Digital Micromirror Device (DMD) -- Texas Instruments ~ 1 million DMD s on a chip M. C. Wu 3 Schematic of TI s DMD M. C. Wu 4

3 Principle of Projection System Using DMD M. C. Wu 5 Digital Micromirror Device (DMD) Texas Instruments Top View of DMD L. Hornbeck, Electronic Imaging, 1997 M. C. Wu 6

4 MEMS-Based Projection Display High brightness High contrast Grey scale achieved by digital modulation (DMD) or analog control (SLM) Compact, light weight, low power Particularly attractive for portable system M. C. Wu 7 Early Development of MEMS Spatial Light Modulator for Display Applications M. C. Wu 8

5 Westinghouse Mirror-Matrix Device Thomas, Westinghouse, 1975 Surface micromachined Built on SOS (Sapphire) Addressed by electron beam Deflect up to 4 Contrast ratio: 1 to 1 Difficulty in HNA release etch Limited resolution in electron beam R. N. Thomas, The mirror matrix tube: A novel light valve for projection display, IEEE T-ED, Vol. ED-, pp , M. C. Wu 9 D Display with Electrostatic Cantilever Light Modulator Array Petersen, IBM, element cantilever light modulator is used in conjunction with a galvanometer to scan the modulated beam across the screen Petersen, K. E., Micromechanical light modulator array fabricated on Silicon, Appl. Phys. Lett., vol. 31, pp.51-3, M. C. Wu 1

6 Silicon Cantilever Light Modulator Petersen, IBM, 1977 SiO structural layer Si sacrificial layer P ++ Doped stop etch layer Cantilever beams biased by voltage Electrically actuated Individually addressable Pull-in observed Petersen, K. E., Micromechanical light modulator array fabricated on Silicon, Appl. Phys. Lett., vol. 31, pp.51-3, M. C. Wu 11 Silicon Torsional Electrostatic Light Modulators Petersen, IBM, 198 Stress in torsion beams is one order of magnitude smaller than fracture stress of single crystal Si Over 1 1 cycles were demonstrated with ± 1 deflection Petersen, K. E., Silicon torsional scanning mirror, IBM J. R&D, vol. 4, pp.631-7, 198. M. C. Wu 1

7 MEMS Optical Display (1) Digital Micromirror Device TM Texas Instruments Ref: Larry J. Hornbeck, Digital Light Processing : A New MEMS-Based Display Technology ( al_light_processing_mems_display_technology.pdf ) M. C. Wu 13 Before DMD was born, there was DMD (Deformable Mirror Device) First reported in 198 Addressed by underlying array of MOS transistor DRAM like architecture Mirror response ~ 5 µs Floating source hold time ~ ms Array size: 18 x 18 Pixel size: 51 µm x 51 µm Air Gap: 6 nm Active area ratio: 3% Ref: Larry Hornbeck, 18x18 Deformable Mirror Device, IEEE Trans. Electron Devices, p. 539, 1983 M. C. Wu 14

8 Early DMD s Hornbeck, Deformable Mirror Spatial Light Modulator SPIE Vol. 115, p. 86, 1989 M. C. Wu 15 Cantilever-Beam DMD s Fabricated by anisotropic wet etching Fabricated by spun-on spacer and plasma etch M. C. Wu 16

9 Cantilever vs Torsion Beam DMD s Torsion-Beam DMD Symmetric torsion beam device has amplitude-dominant modulation Cantilever-Beam DMD Balanced flexure device has phase-dominant modulation M. C. Wu 17 DMD Fabrication M. C. Wu 18

10 DMD Features and Requirements Number of moving parts Mechanical motion Lifetime requirement Address voltage Mechanical elements Process Sacrificial layer Die separation Package Testing.5 to 1. million Makes discrete contacts or landings 45 billion contacts per moving part Limited by 5 volt CMOS technology Aluminum Low temp., sputter deposition, plasma etch Organic, dry-etched, wafer-level removal After removal of sacrificial spacer Optical, hermetic, thermal vias High-speed electro-optical before die separation L. Hornbeck M. C. Wu 19 Fabrication Process for Basic DMD Aluminum alloy-based surface-micromachining process ~3 µm organic sacrificial layer (it also planarize the surface) Hinge: Aluminum alloy, typically 5 to 1 angstrom thick Mirror: Aluminum alloy, typically 3 to 5 angstrom thick Dry releasing in isotropic plasma etching M. C. Wu

11 Digital Micromirror Device (DMD) Texas Instruments M. C. Wu 1 Detailed Layer Structure of DMD L. Hornbeck, Electronic Imaging, 1997 M. C. Wu

12 Fabrication of DMD Use aluminum instead of polysilicon as structural material Completely compatible with CMOS Use DUV-hardened photoresist for sacrificial material Dry releasing in Plasma Etcher to reduce stiction Aluminum alloys are used to improve performance Al mirror may contains a small fraction of Cu and Si One report mentioned.% Ti, 1% Si is added to Al hinges. Al compounds for anti-creep were discussed. DMD superstructure built on CMOS memory (SRAM) circuit 6 photomask layers M. C. Wu 3 Fabrication Process Flow M. C. Wu 4

13 TI s Digital Micromirror Devices (DMD) (Texas Instruments, Digital Micromirror Device TM ) M. C. Wu DMD Analysis M. C. Wu 6

14 (1) Energy Domain Model Voltage-controlled actuation Co-energy Calculate capacitance Q C = V Electrostatic potential distribution: Electric field distribution: r E = V ˆ θ rθ φ( θ ) 1 ( CV * W θ) = θ V 1 θ = r r Total charge on electrode: Q = E da electrode ε M. C. Wu 7 Calculation of Capacitance r r ε V Q = ε E da = electrode θ ε Vw P x1 = ln θ P ( x1 + L) P x1 P ( x1 + L) x 1 1 tanθ ε Vw g = ln θ + x 1 L 1 tanθ g width( r) dr r Assume constant width Using P = g / tanθ M. C. Wu 8

15 Approximate Solution: Stable Angle and Pull-In Voltage Cubic polynomial fit of capacitor: Real solution Pull-in Voltage C( θ ) = C() (1 + a θ + a θ ) kθ θ = 3a C() V 3 1 * C() V 3 W ( θ) = (1 + a1θ + a3θ ) * W ( θ) C() V τ = = ( a1 + 3a3θ ) = kθθ θ ± k a 3a a V PI θ 1 3C() V 3 3 = 1 4 k () θ a a C 3 kθ 3a3C () V a1 3a3 M. C. Wu 9 () Parallel Plate Model by Hornbeck x z Attractive torque τ = L' ε Va 1 x ( z z) a dx Landing Electrode V Bias Electrode- Bias Electrode-1 L V1 Landing Electrode τ a ε V = a W 1 αβ ln(1 αβ ) + tan θ M α 1 αβ tanθ α = tan θ M Normalized angle L L' β = L Actuator loading factor M. C. Wu 3

16 Restoring Force from Torsion Beam Restoring torque Equilibrium τ S θ C = C: torsion compliance τ a + τ S = Solve α (normalized ( angle) for any given voltage M. C. Wu 31 Graphic Solution of Torsion Mirrors Increasing Voltage C B A Electrostatic Torque Torque Restoring Torque Angle α The solution is the intersection point of the two curves corresponding to electrostatic torque and spring restoring torque Low voltage (Curve A): two intersection points but only the smaller angle solution is stable Critical voltage (Curve B): the two curves are tangential and there is only one intersection point called pull-in angle or snap-down angle High voltage (Curve C): there is no intersection no stable solution M. C. Wu 3

17 Transfer Characteristics of Torsion Mirrors Bistable Regime Angle Analog Regime Pull-in voltage (or Stiffness Voltage) 3 z εwl C V S = 3 Voltage Pull-in Voltage M. C. Wu 33 Temporal Response of DMD Analog operation Resonant frequency ~ 5 khz Digital operation Step response time ~ 1 µs M. C. Wu 34

18 DMD Addressing M. C. Wu 35 Addressing Scheme for DMD L. Hornbeck, Electronic Imaging, 1997 M. C. Wu 36

19 DMD Address and Reset Sequence All memory cells under DMD has been loaded All DMD s reset in parallel Allow mirror to release and begin to rotate Loading memory cells for new mirror positions Bias turned on to latch the mirrors in 1 or -1 Field applied to yoke and mirrors M. C. Wu 37 DMD Bias Cycles V=19 =5 V b =4 = V=4 V=4 = V b =4 =5 V=19 V=33.5 =7.5 V b =- 6 = V=6 V=6 V=33.5 = V b =- 6 =7.5 V= V=7.5 V=7.5 V= =7.5 V b =7.5 = = V b =7.5 =7.5 V=16.5 =7.5 V b =4 = V=4 V=4 = V b =4 =7.5 V=16.5 V=19 V=4 V=4 V=19 =5 V b =4 = M. C. Wu 38 = V b =4 =5

20 Bias and Switching Waveforms M. C. Wu 39 Electrostatic Torques on DMD M. C. Wu 4

21 SEM of DMD before Deposition of Mirror Spring Tip to reduce stiction A reset pulse is applied to the mirror and yoke, causing the spring tip to flex. As the spring tips unflex, the reaction force producing a reliable release from the surface. L. Hornbeck, Electronic Imaging, 1997 M. C. Wu 41 Efficiency of DMD Projectors Light Projector Efficiency DMD Pixel Efficiency L. Hornbeck, Electronic Imaging, 1997 M. C. Wu 4

22 Grey Scale of DMD Projector: Pulsewidth Modulation Technique L. Hornbeck, Electronic Imaging, 1997 M. C. Wu 43 DMD Systems M. C. Wu 44

23 Projection Display Using Digital Micromirror Display (DMD) M. C. Wu 45 DMD Projection System with DMD Chips M. C. Wu 46

24 DMD Projection System with 3 DMD Chips M. C. Wu 47 Schematic of Projection Display System with 3 DMD s L. Hornbeck, Electronic Imaging, 1997 M. C. Wu 48

25 Micromirror Array with Vertical Spring Souel National University, Korea Spring hidden under mirror Large reflecting area, high fill factor (84% achieved, 91% theoretical) Simpler fabrication process One mask One shadow evaporation 5µm x 5 µm Al micromirror Pull down voltage = 8 V Response time = µsec (with 9 pplied) Resonant frequency = 11 khz M. C. Wu 49